The present invention relates to a fuel injector.
A fuel injector is discussed in German Published Patent Application No. 195 34 445. The fuel injector includes a valve housing, configured in two parts, in which a valve needle is guided in axially movable fashion. The valve housing has at one end a fuel connector through which fuel is conveyed to the fuel injector. At the other end, the valve needle coacts with the valve housing to constitute a sealing fit, the valve needle being held in the closed position by way of a return spring. To actuate the valve needle, the latter is equipped at the inflow end with a pressure shoulder which coacts with a piezoelectric actuator and is immovably joined to the valve needle. Upon actuation of the valve needle, the actuator acts against the force of the return spring.
Some disadvantages that may be associated with the above fuel injector are as follows:
Since the valve needle is immovably joined to the pressure shoulder, and the valve needle is guided at the spray discharge end, and the pressure shoulder at the inflow end, movably in the valve body, a large inert mass, made up of the mass of the valve needle and the mass of the pressure shoulder, must be actuated in order for the fuel injector to be opened and closed by the actuator and the return spring, respectively. In addition, the two guides provided for axially movable guidance of the valve needlexe2x80x94for the valve needle in the spray-discharge end and on the pressure shoulder at the inflow endxe2x80x94must be matched to one another, the result being that production of the fuel injector is relatively complex, and the fuel injector is susceptible to warping or distortion of the valve needle and/or the valve housing.
Since the return spring also returns the actuator in order to close the fuel injector, the closing motion of the valve needle is not decoupled from the closing motion of the actuator.
As a result of the large mass (made up of the mass of the valve needle and the mass of the pressure shoulder) to be actuated by the return spring, bouncing and therefore unintentional additional spray discharge of fuel occur upon closure of the fuel injector. Another result of this is increased wear on the fuel injector, and thus a shorter service life.
In addition, the fact that guidance of the valve needle is rigid and permanently defined means that the position of the valve needle in the sealing fit is permanently defined, the result being that the valve needle cannot center itself on a sealing fit that deviates from the ideal position as a result of production factors or wear. This results in inhomogeneous and increased wear on the valve needle in the region of the sealing fit, in particular in a degradation in the sealing of the sealing fit of the fuel injector in the closed position, and a change in the geometry of the discharged stream of fuel.
The fuel injector according to an exemplary embodiment of the present invention is believed to have the advantages that it yields a low-wear, reduced-friction design. The fuel injector is moreover almost bounce-free, so that upon actuation of the fuel injector, the duration of the spray discharge operation and the quantity of fuel discharged can be specified in defined fashion.
In an exemplary embodiment, the valve needle is guided in axially movable fashion by a valve needle guide at only one point. In particularly advantageous fashion, the valve needle is small and has low mass.
In an exemplary embodiment, the valve needle rests at one of its end faces against a swirl disk. As a result, the valve needle is guided coaxially with respect to the axis of the fuel injector, thus resulting in homogeneous energy transfer by the valve needle onto the sealing fit, and homogeneous wear in the region of the sealing fit.
In an exemplary embodiment, the valve needle guide and/or the swirl disk have orifices for the passage of fuel. This yields a simple physical design for passage of the fuel.
In an exemplary embodiment, a gap that widens in the radial direction toward the valve axis is formed between the needle collar of the valve needle and the collar of the needle driver. A liquid cushion formed between the collar of the needle driver and the needle collar can thereby be quickly displaced, the result being that the liquid cushion has no influence on switching time and that, in particular, shorter switching times are made possible.
In an exemplary embodiment, the needle driver has at least one orifice or bore for the passage of fuel. The interior of the needle driver can thereby serve as a fuel conduit, the fuel being directed out of the interior of the needle driver through the orifice toward the sealing fit.
In an exemplary embodiment, the orifice is formed by at least one slit in the needle driver extending in the axial direction. The shape of the orifice is thereby adapted to the flow direction of the fuel.
In an exemplary embodiment, the needle driver has an opening with radial enlargements at its end toward the needle collar, which overlap the adjacent end surface of the needle collar to form flow-through windows. The fuel can be passed through the flow-through windows that are created.
Alternatively, the needle driver has a circular opening at its end toward the needle collar, and the end surface of the needle collar is of polygonal configuration, so that the end surface of the needle collar is partially overlapped by the opening of the needle driver to form flow-through openings. As a result, no further design changes to the needle driver are necessary, and flow-through openings that are arranged in a manner favorable to flow are created. In addition, any liquid cushion formed between the collar of the needle driver and the needle collar can be rapidly displaced, the result being that the liquid cushion has no influence on switching times and, in particular, that shorter switching times are made possible.
In an exemplary embodiment, the return spring is braced, at the end facing away from the needle collar, against an adjusting element, the adjusting element being joined to the needle driver. As a result, on the one hand the return spring can be preloaded in a defined manner that is simple in terms of production engineering. On the other hand, the return spring defines only the closing force of the fuel injector when the fuel injector is closed. The forces required to open and close the fuel injector can then be defined by the actuator and by at least one further spring.